This invention relates to the charging of lithium-ion batteries and more specifically to the determination of the optimum charging currents for lithium ion batteries.
Lithium-ion secondary batteries are quickly gaining favor due to their lightweight and high energy density. Before now, chargers were built to charge the battery under relatively low currents for long periods of time, so as to avoid battery heating and electrode damage. Alternately, a relatively high fixed current was used which was known to be in a safe current regime for a newly manufactured battery. As the battery ages, or if a different battery is placed on a given charger, the fixed current may exceed the safe current regime, at which time battery heating and electrode damage might occur.
In order to save time in the charging of lithium-ion batteries and to still assure the safety of the charging technique, it is desirable to be able to determine an optimum charging current. Further, to improve the charging techniques it is desirable to be able to have knowledge of the state of charge (SOC) and capacity of the battery being charged. Priorly in the art, SOC and capacity were obtained by fully charging and discharging the battery in question and tracking the corresponding battery voltage vs. energy input/output. This charging and discharging of each battery to be charged again requires time but also uses energy.
A need therefore exists for methods that can determine the optimum charging current for safely yet time-effectively charging a specific lithium-ion battery. There also exists the related need for determining the SOC of the battery as part of the determination of the optimum charging current determination without the need to charge and discharge the specific battery to be charged.
We have found that the optimal charging current of a lithium-ion battery can be determined by performing a series of charging experiments utilizing varying initial charging currents, and by then recording cell voltage, cell temperature, and charging time. Specifically, we have found that the controlling variable in determining the optimum charging current is the state of charge (SOC) of the battery. The storage capacity of a battery is typically specified in Amp-Hours (Ah), where 1 Ah=3600 coulombs, and the state of charge (SOC) of a battery is defined as the actual charge stored in a given battery capacity divided by the storage capacity of that battery.
In accordance with one aspect of our invention, look-up tables or algorithms for each type of lithium-ion battery are prepared and stored in a computer or database. These look-up tables co-relate either open-circuit voltage vs. state of charge or ramp-peak current vs. state of charge for each type of lithium-ion battery, or include both co-relations. Our invention includes both the method steps for creation of the look-up tables as well as the use of the look-up tables in determining the optimum charging current for a particular lithium-ion battery.